1. Field of the Invention
The present invention relates to a control circuit for servo-motors and, more particularly, to a control circuit for servo-motors applied to an automatic diaphragm means for cine-cameras.
2. Description of the Prior Art
As the control circuit for servo-motors of this kind, a circuit arranged as shown in FIG. 1 is known. That is, as shown in FIG. 1, a DC amplifier 5 is connected to the output terminal of a Wheatstone bridge circuit comprising a photoconductive element 1 such as CdS and resistors 2, 3 and 4. Besides, a servo-motor 8 is connected to the output terminal of the DC amplifier 5 through transistors 6 and 7 for which the polarity is different from each other. Thus, said servo-motor 8 is rotated in forward or reverse direction according to the polarity of the unbalanced voltage which occurs at the output terminal of said bridge circuit in order to open or close diaphragm blades arranged in front of the photoconductive element 1 until the bridge circuit reaches the balanced state. On the other hand, by rotation of the motor 8 at that time, a photographing lens diaphragm means of the camera is controlled in order to control the exposure value always to the proper amount according to the intensity of light from the object to be photographed.
The above-mentioned control circuit, however, has the following disadvantages. That is, gamma (.gamma.) of the photoconductive element 1 has to be constant. Besides, when exposure informations such as the sensitivity of the film, film feeding speed, etc. are to be introduced to resistors 3 and 4, the operating range of the DC amplifier 5 will be exceeded if too much information is introduced and, therefore, the information which can be introduced as above is limited. Besides, in case that CdS is used as the photoconductive element 1, the delay in response to CdS tends to become a cause of hunting phenomenon because of fluctuation of the amount of incident light from the object, when exposure information such as sensitivity of the film etc. is set, especially when the resistance value of CdS is large (when the light intensity of the object is low).